TY - JOUR
T1 - A Multireference Ab Initio Study of the Diradical Isomers of Pyrazine
AU - Scott, Thais
AU - Nieman, Reed
AU - Luxon, Adam
AU - Zhang, Boyi
AU - Lischka, Hans
AU - Gagliardi, Laura
AU - Parish, Carol A.
N1 - Publisher Copyright:
© 2019 American Chemical Society.
PY - 2019/3/14
Y1 - 2019/3/14
N2 - Three diradical pyrazine isomers were characterized using highly correlated, multireference methods. The lowest lying singlet and triplet state geometries of 2,3-didehydropyrazine (ortho), 2,5-didehydropyrazine (para), and 2,6-didehydropyrazine (meta) were determined. Two active reference spaces were utilized. The complete active space (CAS) (8,8) includes the σ and σ∗ orbitals on the dehydrocarbon atoms as well as the valence π and π∗ orbitals. The CAS (12,10) reference space includes two additional orbitals corresponding to the in-phase and out-of-phase nitrogen lone pair orbitals. Adiabatic and vertical gaps between the lowest lying singlet and triplet states, optimized geometries, canonicalized orbital energies, unpaired electron densities, and spin polarization effects were compared. We find that the singlet states of each diradical isomer contain two significantly weighted configurations, and the larger active space is necessary for the proper physical characterization of both the singlet and triplet states. The singlet-triplet splitting is very small for the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers (+1.8 and -1.4 kcal/mol, respectively) and significant for the 2,5-didehydropyrazine (para) isomer (+28.2 kcal/mol). Singlet geometries show through-space interactions between the dehydocarbon atoms in the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers. An analysis of the effectively unpaired electrons suggests that the 2,5-didehydropyrazine (para) isomer also displays through-bond coupling between the diradical electrons.
AB - Three diradical pyrazine isomers were characterized using highly correlated, multireference methods. The lowest lying singlet and triplet state geometries of 2,3-didehydropyrazine (ortho), 2,5-didehydropyrazine (para), and 2,6-didehydropyrazine (meta) were determined. Two active reference spaces were utilized. The complete active space (CAS) (8,8) includes the σ and σ∗ orbitals on the dehydrocarbon atoms as well as the valence π and π∗ orbitals. The CAS (12,10) reference space includes two additional orbitals corresponding to the in-phase and out-of-phase nitrogen lone pair orbitals. Adiabatic and vertical gaps between the lowest lying singlet and triplet states, optimized geometries, canonicalized orbital energies, unpaired electron densities, and spin polarization effects were compared. We find that the singlet states of each diradical isomer contain two significantly weighted configurations, and the larger active space is necessary for the proper physical characterization of both the singlet and triplet states. The singlet-triplet splitting is very small for the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers (+1.8 and -1.4 kcal/mol, respectively) and significant for the 2,5-didehydropyrazine (para) isomer (+28.2 kcal/mol). Singlet geometries show through-space interactions between the dehydocarbon atoms in the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers. An analysis of the effectively unpaired electrons suggests that the 2,5-didehydropyrazine (para) isomer also displays through-bond coupling between the diradical electrons.
UR - http://www.scopus.com/inward/record.url?scp=85062882619&partnerID=8YFLogxK
U2 - 10.1021/acs.jpca.8b12440
DO - 10.1021/acs.jpca.8b12440
M3 - Article
C2 - 30779575
AN - SCOPUS:85062882619
SN - 1089-5639
VL - 123
SP - 2049
EP - 2057
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 10
ER -